1. Field of the Invention
The present invention relates to electro-optic materials. In particular, the present invention relates to methods of poling electro-optic polymer materials for use in optical devices.
2. Discussion of the Background
Electro-optic (EO) materials play a key role in optical communication systems, allowing for chirp free high speed modulation. EO properties are exhibited by both inorganic materials and polymers. EO polymers have advantages over inorganic materials with respect to bandwidth, EO activity, cost and processability.
The low dielectric constants of EO polymers allow for extremely high bandwidth devices, as evidenced by reports of EO polymer modulators with bandwidths of more than 100 GHz.
EO polymers with Pockel's coefficients, r33, of 262 pm/V have been reported, which may make possible in the near future low-cost, high bandwidth, EO modulators with less than 0.5 V drive voltages and 6 dB insertion loss. Such devices are of interest for analog radio frequency (RF) links, satellite communications and fiber to the home. These EO polymers are also expected to have application in optical packet switching, reconfigurable optical interconnects, wavelength conversion, and terahertz generation, among other areas.
Since EO polymers are soluble in organic solvents, they can be deposited as thin films by spin coating, eliminating the need for single crystal growth techniques. Furthermore, they can be integrated with semiconductor technology without the need for crystal lattice matching.
Most polymers are not electro-optic. To provide electro-optic properties to a polymer, chromophore molecules can be attached in a side-chain fashion to the backbone of a polymer chain. Alternatively, chromophores can be doped into a polymer matrix to provide a guest-host EO polymer.
In order to have a second order optical nonlinearity in a guest-host EO polymer, the electric dipoles in the chromophores need to be oriented in a preferred direction by an electric field in what is known as “poling”. Poling can be done by applying a DC voltage at a temperature near the Tg of the host polymer and then cooling the polymer to room temperature with the field still applied to lock in the order to the degree possible by the governing statistical mechanics.
Two common techniques for poling are “contact poling” and “corona poling”. In contact poling, voltage is applied directly to metallic electrodes contacting opposite sides of an EO film. In corona poling, a large electric field is applied between a needle positioned above an EO film and a ground plane on the other side of the EO film. The large electric field creates a corona discharge in a gas between the needle and the EO film. Ions build up on the surface of the EO film and create a very strong electric field across the EO film that poles the film.
When used in optical devices, EO polymers are often clad with a variety of materials. It is well known that the choice of cladding layers in poled EO polymer based devices has a major impact on device performance. Sol-gels have been studied extensively as materials for passive waveguide devices. U.S. Pat. Nos. 6,937,811 and 7,206,490 disclose EO waveguide devices that include a poled EO polymer core and, surrounding the core, claddings of polymers or organically modified sol-gels.
To first order, the strength of the EO coefficient, r33, resulting from poling is directly proportional to the strength of the applied electric field. Thus, it is important to be able to apply as high an electric field as possible to an EO film during poling.
However, the EO coefficients that can be achieved by poling have been limited by the onset of dielectric breakdown in films.